1. Field of the Invention
The present invention relates to machine controls, and more particularly to a switch assembly for initiating or changing a mode of operation of a machine.
2. Description of the Prior Art
It has long been known to use a spaced pair of manually operable switches (often of the type known as palm buttons) to activate the operational modes of machines such as stamping presses, riveters, tube benders, spot welders or the like. The purpose of the spaced pair of manually operable switches is to ensure that both hands of the worker are positioned away from the machine prior to operation. For this purpose of safety, the switches of such a machine must be operated simultaneously and must be spaced sufficiently far apart or otherwise suitably obstructed to prevent simultaneous actuation of the switches with a single hand. The Federal Occupational Safety and Health Administration, of course, sets the criteria which must be met by such machine operating equipment, to ensure adequate worker safety.
A schematic of a conventional control circuit for such machines is shown in FIG. 4. The circuit includes a pair of palm buttons 110 which each include a spaced pair of electrical contacts 112 and 114. One contact 112 is disposed in a first limb 116 of a parallel machine operating circuit 118, while the other contact 114 is disposed in a second limb 120 of the operating circuit 118. The circuit 118 includes control means 122 for instituting alternative operative modes of a machine 10, as well as anti-tiedown logic circuitry, as described below. The connection between the machine 10 and logic and control circuitry 122 is shown only schematically in FIG. 4, since this connection is well known in the art. For example, the control means 122 can first direct the machine to load a part and then direct the machine to operate on the part (not shown).
The palm buttons 110 are spring-biased or pneumatically biased, so that in the absence of the application of an exertive contact by the operator, the contacts 112 complete the limb 116 and cause the control and logic circuitry means 122 to operate the machine 10 in its first mode of operation. When the operator applies exertive contact to the palm buttons 110, the contacts 112 are moved out of connection to the limb 116, and the contacts 114 are subsequently moved into connection to the limb 120. This causes the control means 122 to operate the machine 10 in its second mode of operation.
It is undesirable to use simple single throw switches to replace the palm buttons 110 because this can obviate the protection given to the worker when the second switch is employed. If one switch is left in an operative position while the other switch can be operated by a single hand of a worker, the worker's free hand can become entangled with operative portions of the machine, sometimes resulting in potentially fatal injury. Indeed, it is for this reason that OSHA regulations generally require that existing machines already incorporate anti-tiedown logic circuitry that is, a control to define and limit the intervals of and between activation of the spaced switches so that, in order for actuation to be considered substantially simultaneous (and thereby permit machine operation), the switches typically must both be actuated each time machine operation is desired, and both also actuated within one second of each other. In a known manner this circuitry thus prevents machine operation if one switch is attempted to be continuously actuated, by being tied down. This conventional anti-tiedown logic circuitry and machine control means is not by itself part of the present invention.
While the use of a spaced pair of palm buttons has been adequate for the intended purpose of safety, such buttons are subject to the drawback that the worker must apply significant exertive contact to the palm buttons, in order to achieve actuation of them. A substantial number of machine operators are now recognized as being subject to fatigue from the repetitive application of exertive contact against conventional palm buttons. Indeed, operators are increasingly experiencing inflammation of the tendons, due to this repetitive and forceful action of the fingers and wrists. This inflammation of the tendons of the fingers and wrists is known as Carpal Tunnel Syndrome. The ailment has increased worker fatigue and complaints, has decreased productivity and has increased the frequency of disability claims under the workmen's compensation laws and programs. These effects are injurious to the worker and substantially increase the costs associated with operation of the machine in question.
Additionally, palm buttons and like devices are subject to unacceptably great wear and breakage, due in part to the frictional movement of the members making them up, for example, such as the shaft and bore of a conventional palm button. Additionally, contacts such as the button contacts 112 and 114 described above are subject to bending upon repeated mechanical manipulation and impactment against the circuit contact which they abut.